The present invention relates to an ink-jet printer head of the piezoelectric type.
An ink-jet printer head of the thermal-bubble type is conventionally used to jet the ink onto a medium to form an image thereon. The printer head of the type typically generates a large driving force. i.e. about 40 atmosphere pressures, onto the ink droplet being jetted. At the moment the ink droplet leaves the printer head, a droplet trailing phenomenon is observed. Therefore, ink-jet printer of the type wastes ink, and has difficulty in controlling the desired shape and size of the ink droplet. In addition, lower resolution printing quality is also the drawback with the thermal-bubble type ink-jet printer.
The piezoelectric type is another category of the ink-jet printer head which utilizes a piezoelectric ceramic plate as an actuator for driving the ink. The driving force of such type is about 4 atmosphere pressures, which is much smaller than one generated by the thermal-bubble type. Due to the characteristic of driving mechanism, the size of the ink droplet is smaller and the droplet trailing phenomenon is substantially reduced. In addition, the piezoelectric type printer head saves ink and has a higher resolution compared with the thermal-bubble ink-jet type.
The characteristics of the piezoelectric ceramic plate is introduced in the following by referencing FIG. 1. As well known in the arts, the piezoelectric ceramic plate is made with one predetermined polarization direction. For piezoelectric material whose polarization direction is d31, the deformation of the piezoelectric material will be in X direction, when an electric field is applied in Z direction. On the other hand, for piezoelectric material whose polarization direction is d33, the deformation of the piezoelectric material will be in Z direction, when the applied electric field is in Z direction. Two well known conventional approaches are used to operate the piezoelectric type printer head. The first one involves utilizing a multi-layer piezoelectric ceramic plate as an actuator to jet the ink as shown in FIG. 2. Referring to FIG. 2, the multi-layer, i.e. 8 layers, piezoelectric ceramic plate 20 is disposed in a housing with the bottom end fixed and the upper end free to move. The polarization direction of each layer of the piezoelectric ceramic plate 20 is d33. The positive electrodes for each layer within the multi-layer ceramic plate 20 together form a comb configuration denoted as 100. The negative electrodes for each layer within the multi-layer ceramic plate 20 together form a comb configuration denoted as 200.
Initially when a first voltage is applied across the positive and negative electrodes, the electric field generated will make each layer deform and cause the multi-layer piezoelectric plate 20 to move downwards. The rubber pad 21 moves downwards accordingly. The space of the ink tank 23 becomes larger and the ink flows from the ink container 24 into the ink tank 23 via the passage 25. Afterwards when a second voltage is applied across the positive and negative electrodes, the direction of the electric field generated will be opposite, and each layer deforms in the opposite direction and causes the multi-layer piezoelectric plate 20 to move upwards. The rubber pad 21 moves upwards accordingly. The ink tank 23 will become smaller, and the pressure inside the ink tank 23 will force the ink to be jetted from the ink tank 23 via the outlet 22.
In the structure of FIG. 2, the multi-layer piezoelectric ceramic plate 20 is positioned under the outlet 22 with the upper end moves in a vertical direction. The amount of the displacement xcex94X of the upper end of the multi-layer piezoelectric ceramic plate 20 is calculated by the following equation: xcex94X=d33*V*n, wherein d33 is the piezoelectric parameter, V is the voltage applied across two electrodes, and n is the number of the layers within the multi-layer piezoelectric ceramic plate 20. Due to its multi-layer structure, the multi-layer piezoelectric ceramic plate in FIG. 2 has a larger displacement when applied with a voltage, and results in a larger driving force to the ink. However, the manufacturing of multi-layer piezoelectric ceramic plate 20 and the electrodes is difficult and costly.
The second approach performs the function through another way. The walls of the ink tank are formed by piezoelectric ceramic segments. When the walls of the ink tank are applied with a voltage, the shape of the ink tank will be changed and thus the ink will be jetted out of the ink tank. FIG. 3a shows a cross-sectional view of the structure in which the side walls of the ink tank 302 deforms in response to the voltage applied across the corresponding electrodes. The shown cross section is perpendicular to the longitudinal dimension (into the paper) of the ink tanks 301, 302, 303. The structure includes a plurality of single-layer piezoelectric ceramic segments 321, 322, 323, 324 which are formed by a diamond cutting process on a single sheet of piezoelectric ceramic plate. After the cutting procedure, corresponding side walls of two successive piezoelectric ceramic segments, i.e. 322, 323, constitute one ink tank 302 therebetween. The electrodes 39 on the inner surface of each ink tank are respectively formed by an electrodeless nickel plating process. A sheet of glass or ceramic plate 34 is covered and connected onto the upper surface of the piezoelectric ceramic segments to totally enclose the tank space. Two voltages A, B shown in FIG. 3(b) are applied across the respective electrodes to create corresponding deformation as desired. As a result, the right side wall of the tank 302 deforms rightwards and the left side wall of the tank 302 deforms leftwards. Therefore, the size of the ink tank 302 increases due to the deformation. The space of the ink tanks 302 increases, and the ink will be drawn from an ink container (not shown) into the ink tank 302. Afterwards, the voltage A drops sharply to a negative value and the voltage B elevates sharply to a positive value. Due to this opposite action, the dimension of the tank 302 decreases due to the deformation of the piezoelectric ceramic segments 322, 323 in a reverse direction. As the space of the ink tank 302 decreases, the ink is jetted from the ink tank 302 via an outlet 31. The plastic substrate 38 is made of soft and resilient material which also helps the ink tank 302 generate the driving force. Since the electrodeless plating process is used to manufacture the electrodes 39, its endurance against the ink erosion is enhanced. However, this second approach of the piezoelectric type printer head is complex in structure and in manufacturing. More details regarding the second approach disclosed in FIG. 3(a) can be found in U.S. Pat. No. 5,327,627.
The main object of the present invention is to provide a ink-jet printer head of the piezoelectric ceramic type which has a simple structure and is easy to manufacture.
In the present invention, the printer head includes a deformable polymer membrane, an ink tank and a dual-layer piezoelectric ceramic plate. The dual-layer piezoelectric ceramic plate is mounted on the deformable polymer membrane which functions to apply a perturbation force to the ink within the ink tank. The dual-layer of the piezoelectric ceramic plate includes an top layer and a bottom layer, both of which have same polarization direction. One end of the piezoelectric ceramic plate is fixed to the membrane and the other end is free to vibrate. When a voltage is applied across two electrodes at the fixed end of the dual-layer piezoelectric ceramic plate, the free end of the dual-layer piezoelectric ceramic plate vibrates. Through the deformable membrane, a perturbation force is created and drives the ink to be jetted outside the ink tank via an outlet.